Merge: Remove memory leak in Ropes

Early on, the `to_s` of a RopeBuffer was calling `dump_buffer`, which like to double the size of the buffered part after persisting.

By doing do, a simple program like the one submitted along with this PR was consuming an awful lot of RAM, which has the undesirable side-effect of freezing my machine after a few milliseconds of run.

By replacing it with the more memory-friendly `persist buffer`, all my troubles seem so far away !

Pull-Request: #1598
Reviewed-by: Jean Privat <jean@pryen.org>
Reviewed-by: Romain Chanoir <chanoir.romain@courrier.uqam.ca>

diff --cc lib/standard/text/ropes.nit
Simple merge